Naval architects at Robert Allan Ltd designed new LNG-fuelled escort tugs for Norwegian shipowner Østensjø Rederi using the latest software and knowledge from classification society Bureau Veritas (BV). The vessels were designed to support operations at Statoil’s Melkøya LNG production terminal near Hammerfest in northern Norway.
In this concluding episode of a three-part feature, Robert Allan project manager Mike Phillips explains how the RAstar tug hullform and auxiliary equipment were designed for these tugs. In part one, Mike Phillips described the challenges of designing these vessels. In part two he discussed the method Robert Allan took for designing the LNG system on these tugs to specific owner, charterer, flag state and class requirements.
However, some would say the most important element to a tug design is ensuring the hullform is safe, stable and efficient. Robert Allan used a RAstar tug hullform for the LNG-fuelled tugs. It also took careful consideration of the motions and accelerations of the LNG in the tank.
Although the dual-fuel engines allow for the seamless transition from LNG to diesel with the engines running, it is still a design objective of the tug owners to operate on gas in all operating conditions. To achieve this, a RAstar tug hullform was chosen to significantly reduce tug motions at sea, Mr Phillips explained. The LNG tank is orientated in the longitudinal direction to minimise any risk of a pressure breakdown in the tank due to excessive sloshing and mixing of the liquid and gas. Additional internal baffling was installed, and the tank supports and internal structure were designed to the maximum possible acceleration loads under the IGC Code.
The performance requirements for these 40m tugs were high, which brought further challenges. These included indirect escort forces of 155-160 tonnes at 10 knots, a bollard pull of 100 tonnes, and a top speed of 15 knots. To be sure that these performance requirements will be met, extensive hydrodynamic simulations were performed during the design process using both computational fluid dynamics (CFD) and an extensive model testing programme.
Robert Allan began with the standard RAstar hullform, which is already known for its ability to generate superior escort forces with a high degree of stability. Multiple hull and underwater appendage refinements were examined in CFD to find a solution that balanced escort force generation and stability and safety capabilities, while also providing enough room in the hull for the LNG tank and system. The concept was then model tested at the Vienna Model Basin, with tests including resistance and propulsion, escort performance, and seakeeping.
Since the tugs will be serving an LNG terminal, the requirements also called for them to be fitted with a foam-capable external fire-fighting system with the capacity to deliver streams of water at rates of 2,400 m3/hr to a range in excess of 120m. This is to meet the class FiFi 1 standard. The tugs also have fendering systems specially designed to limit fender contact pressures to 20 tonnes/m2 or less. Other owner requirements included towing winches enclosed in the deckhouse for protection from the harsh local elements, clean and efficient working decks with integral heating systems to keep them free of ice, and comfortable living spaces for up to eight crew members.
Fitting all of this into a 40m tug, in addition to the large LNG system, propulsion machinery components, and oil recovery equipment, presented a significant design challenge, said Mr Phillips. Nevertheless, by working in close collaboration with the owner and BV, a customised solution was found that met all of the owner and charter requirements. The deckhouse maximises the available working deck areas by integrating the accommodation, winch enclosures, and towing staples, while the sloping and curved surfaces promote optimal and commanding sightlines throughout from the master's position in the wheelhouse. The distinctive superstructure design also helps to avoid the build-up of snow and ice. Visual obstructions are kept to a minimum with working decks completely clear of LNG system piping, and minimalistic exhaust casings.
Østensjø Rederi selected Astilleros Gondan Shipyard in northern Spain to build the three dual-fuel LNG terminal escort tugs. Construction commenced in the fourth quarter of 2015, with delivery of all three tugs expected in time for the beginning of operations in the first half of 2017. BV will continue to be a key player in the project by surveying construction and eventual classification of the tugs. With a tailored and innovative design solution to a unique set of complex challenges, these tugs will set a high bar for not only LNG-fuelled tugs worldwide, but for high-performance escort tugs as well.
Mike Phillips is a project manager and naval architect at Robert Allan Ltd